Most car engines are pre-mixed spark-ignition gasoline-type engines, or compression-ignition diesel-type engines. HCCI combines both techniques; it utilizes a pre-mixed air-fuel charge, as with spark-ignition engines, but ignites the mixture using compression, rather than a spark ignition mechanism such as a spark plug, to raise the temperature of the air-fuel mixture to the point of combustion. HCCI involves the compression of a pre-mixed air-fuel mixture, resulting in uniform combustion throughout the mixture.
By replacing traditional heterogeneous hot combustion by propagation of a flame-front, with overall, flameless, homogenous combustion (in which the entire, homogenously mixed air-fuel mixture combusts simultaneously), combustion can be achieved at lower temperatures (typically less than 1,300° C.). HCCI engines therefore typically produce lower levels of NOx and particulate matter (soot), while offering greater economic operating efficiency.
Despite the advantages of HCCI, HCCI engines are not widely commercially available. The auto-ignition of the homogenous air-fuel mixture has proven difficult to control and keep stable. Typically, control is attempted using a microprocessor or Engine Control Unit (ECU) which monitors certain operating conditions in order to accurately control the combustion chamber temperature and mixture on each iteration of the combustion cycle (such as the four-stroke Otto cycle). However, the parameters are many (e.g., RPMs, cylinder pressure, ambient air temperature, engine temperature, engine load, atmospheric pressure) and the ECU must be capable of dynamic and precise control of the actuation system (e.g. air intake/exhaust valves), within very short periods of time, in order to achieve HCCI. A typical approach to controlling HCCI combustion is to vary the compression ratio of the combustion chamber in accordance with such factors.
Further, because HCCI requires high compression ratios, stronger engine construction is often required for HCCI engines. This is especially a concern in HCCI where improperly controlled combustion can lead to detonation rather than combustion. If the engine construction is not robust, this can lead to destruction of the engine, as is sometimes seen with detonation events in reciprocating aluminum piston engines. Rotary engines, by virtue of their geometry, are inherently stronger and are therefore less prone to destruction in the event of detonation.
Because of the high pressure conditions, the many factors required to be monitored, and the limited time in which to process and induce the physical adjustments necessary to the actuation system in response to the factors, it is difficult to realize HCCI in typical reciprocating or Wankel™ type-engines. These engines have complex actuation systems, involving, for example, valves for in-take and exhaust, which adds to the number of elements required to be controlled and adjusted in order to achieve HCCI.
In some cases, involving reciprocating or Wankel™ engines, small diameter piston heads (“controlling pistons”) disposed within the combustion chamber have been used. However, because of the relatively small ratio of the surface area of the controlling piston to the relatively large combustion chamber volume, the controlling piston is of little effect and is required to be moved a great extent in order to achieve minimal changes in chamber volume (and therefore in compression ratio). Furthermore, in such cases, the range of compression ratios that can be realized is narrowed as the controlling piston is incapable of reducing the chamber volume to zero. If the controlling piston is increased in size, little space is left in the cylinder head for other required components, such as valves, spark plugs and fuel injectors, and therefore, such solutions result either in an implementation with small valves, or an implementation with small controlling pistons and combustion chambers having a small range of compression ratio variance. In either case, the result is an engine in which it would be difficult to control and maintain HCCI under varying operating conditions.